Polymer electrolyte fuel cells, which are now being expected to serve as a power source for vehicles, have heretofore been known for their own disadvantages that, in a low temperature environment below freezing, moisture around electrodes is frozen which obstructs the diffusion of reactant gas and in turn decreases the electric conductivity of electrolyte membranes. Further, there is another disadvantage that upon activating a fuel cell in such a low-temperature environment, clogging occurs in reactant gas flow channels due to the moisture freezing and the reactant gas is obstructed from advancing to and reaching the electrolyte membranes, which sometimes retard an electrochemical reaction of the reactant gas even though being supplied and hence result in failure to start-up the fuel cell. Furthermore, frozen dew produced in the reactant gas flow channels may blockade gas flow paths.
In view of such backgrounds as the above, as disclosed in Japanese Patent Laid-Open No. 2003-36874, there, has been proposed a fuel cell system having detector for detecting internal temperate of a fuel cell to deactivate a cooling water pump when the internal temperature of the fuel cell is below freezing and to increase the driving amount of the cooling water pump in accordance with a rise in the internal temperature of the fuel cell when the internal temperature thereof is above freezing, in order to prevent generated water from freezing in the fuel cell.